One type of prior art image sensor is an active pixel sensor (APS) image sensor. APS image sensors are typically fabricated using Complimentary Metal Oxide Semiconductor (CMOS) processing technology, and are also typically referred to as CMOS image sensors. CMOS image sensors sense light by converting incident light (photons) into electronic charge (electrons) via the photoelectric effect. Color CMOS image sensors are typically made by coating each individual pixel with a filter color (e.g. red, green, and blue). CMOS image sensors typically include a photo sensor (e.g., photodiode) and several CMOS transistors for each pixel. Some CMOS image sensors provide integrated analog-to-digital conversion and full timing control on a single integrated circuit.
Existing CMOS image sensors include three-transistor (3T) and four-transistor (4T) pixel implementations. Pixel implementations with more than four transistors have also been developed. The pixel circuits in these image sensors typically include a source follower transistor that is used to buffer the photodiode voltage onto a column line. In CMOS image sensors with 4T pinned photodiode pixel implementations, read noise is typically dominated by the 1/f noise of the source follower transistor. 1/f noise, which is also referred to as flicker noise, has a spectral density that is inversely proportional to frequency (f). The 1/f noise of the source follower transistor is also a factor in 3T pixel implementations, although the 1/f noise is not typically dominant. Rather, in 3T pixel implementations, the read noise is typically dominated by “kTC” noise, which is the noise associated with resetting the pixel to a reset level. Nonetheless, 1/f noise from the source follower transistor provides a significant contribution to the overall noise in 3T pixel implementations.
Existing techniques for reducing the 1/f noise of a transistor include increasing the size of the transistor, or modifying the CMOS process. However, due to pixel size constraints in CMOS image sensors, the size of the source follower transistor cannot typically be increased. And modification of the CMOS process is typically complex and not a viable option.